Ok kiddo, "cmp fat" means comparing two things called "fat". You know that our body has fat, right? It's something we need to have a healthy body, but too much of it is not good for us.
Similarly, when we talk about a computer's memory or storage, we call it "fat". And we use a tool called "cmp" to compare two different sets of data- like two files or two programs.
So in short, "cmp fat" means comparing two different parts of a computer's memory or storage to see how they are different or similar.